The invention relates to a process for adjusting the capacity of a low-temperature rectification unit having a high pressure column and a low pressure column which a fluid is introduced into the high pressure column, and in which liquid from the bottom of the high pressure column is directed into the low pressure column.
Adjustment processes for changing the capacity of a low-temperature rectification unit in which a nominal value is set as a correcting value for the bottom level in the column that operates at higher pressure are known. By changing the nominal value of the bottom level, the unit is adjusted to an altered amount of working fluid. The change in capacity is also called load alteration. In the case of a change in the amount of working fluid accompanied by a load alteration, a change in the ratio of liquid to vapor (L/V ratio) temporarily occurs in the rectification column. This change brings about an undesirable change in the purity of the products. The known adjustment processes therefore have the goal of keeping the L/V ratio as constant as possible so that the products of the rectification unit have the same purity for various types of loads of the unit. This is achieved in all known processes by an adjustment of the bottom level that uses the liquid level as a correcting value. Such adjustment processes are known from, for example, publications EP 0 654 436 or U.S. Pat. No. 3,912,476.
Moreover, it is known to use buffer storage to keep the L/V ratio constant. This method is associated with a considerable structural expense, however.
In addition, it is known that not only in the case of capacity changes must measures be taken to keep the L/V ratio constant, but also in the case of the occurrence of malfunctions. For example, fluctuations in the amount of air in an air separation unit lead to different liquid feeds into the bottom of the column that operates at higher pressure. In the known bottom level adjustment, which keeps the bottom level constant, the altered amount of liquid feed and the altered amount of liquid flow are sent on. That is to say that variable flows pass from the bottom of the column that operates at higher pressure into the column that operates at lower pressure. This has a negative effect in the column that operates at lower pressure, since the variable liquid feed disrupts rectification.
The object of the invention is therefore to make available a process that ensures low-temperature rectification with uniform product purity with load variation, such as also in the case of malfunctions with varying amounts of working fluid.
This object is achieved according to the invention in that the amount of liquid that is removed from the bottom of the column that operates at higher pressure and that is fed to the column that operates at lower pressure is controlled via flow adjustment, whereby a nominal value for the flow adjustment is set to a desired amount of flow, and the liquid level of the liquid at the bottom of the column that operates at higher pressure is permitted to self adjust without a set nominal value corresponding to the amount of liquid that is removed. In this case, as flow adjustment, preferably an FIC (Flow-Indicated Control) unit is used as an adjustment component. This unit usually has a flowmeter, as well as at least one valve that can be actuated and that is connected to the flowmeter via a control line.
The low-temperature rectification unit is preferably a low-temperature rectification unit for air separation. A bottom level indicator (level indicator) is used especially preferably to increase the operating safety, in addition to adjust the flow, and its purpose consists in indicating going above a set maximum value for the bottom level, as well as dropping below a set minimum value. It is advisable for a warning signal to be issued in the event the values described go above or below the permitted range.
A further development of the invention provides that in addition to the column that operates at higher pressure and the column that operates at lower pressure, the low-temperature rectification unit has an argon column with a top condenser, and liquid from the bottom of the column that operates at higher pressure is directed into the top condenser and from the top condenser into the column that operates at lower pressure, whereby the amount of liquid that is removed from the bottom of the column that operates at higher pressure is controlled via flow adjustment, whereby a valve is actuated that determines the amount of fluid to be supplied to the column that operates at lower pressure, and a valve is actuated that can allow the appropriate amount of fluid to flow into the top condenser of the argon column.
It is especially advantageous for the degree of opening of the two valves to be set by means of split-range adjustment. In this case, a distribution of the amount of liquid that is removed from the bottom is carried out. The distribution of the amount is advantageously given in portions to the two valves. In the case of a change in the amount of flow, the valve positions automatically adapt according to the portions that they handle.
Especially advantageously, in the case of an increase in the capacity of a low-temperature rectification unit, no nitrogen-rich liquid that is stored outside of the column that operates at higher pressure is fed into the column that operates at higher pressure. The bottom and the attached network in the column that operates at higher pressure and optionally the top condenser of the argon column are preferably used as liquid buffers in the case of load changes. Keeping the L/V ratio constant is thus facilitated. A combination of the two as buffer storage is also advantageously used.
The invention has the advantage that even with fluctuations in the amount of working fluid, only a constant amount of liquid is removed from the bottom of the column that operates at higher pressure. As a result, the liquid feed remains constant and undisturbed in the column that operates at lower pressure. The rectification in the column that operates at lower pressure remains entirely unchanged. The product purity stays the same. In addition, flow adjustment is a reliable adjustment method that can be carried out simply with few components required.
The change in the amount of working fluid becomes apparent in each case only in a change of the bottom level, which is self adjusting without set nominal values corresponding to the amount of liquid that is removed from the bottom of the column that operates at higher pressure. To increase the reliability, only a maximum value and a minimum value are specified for the bottom level. It is advisable for a warning signal to be issued when the value goes above the specified maximum for the bottom level and when the value drops below the specified minimum. Within these limits, the bottom level of the column that operates at higher pressure can drop or increase, without this having disturbing effects on the rectification.
For the case that a warning signal indicates that the value goes above the specific maximum or goes below the specified minimum for the bottom level, several measures are available that all aim at bringing the bottom level back into the range that is not critical between the maximum and minimum values. All measures have in common the fact that one or more of the fluid streams that flow out of the column that operates at higher pressure or into the latter are changed as regards their throughput.
For example, when the maximum value for the bottom level is exceeded, the amount of liquid removed from the bottom increases or the amount of feed air is reduced. The feed amount of the nitrogen-rich liquid into the column that operates at lower pressure or into a storage tank can also be increased.
In the case the value goes below the minimum for the bottom level in the column that operates at higher pressure, the above-mentioned measures are carried out with opposite signs.